EMI RS61 schematic?

[rock soderstrom]

@ dustbro: brilliant idea! 8)

No doubt, I would vote for the original tubes!  ;D They are not that expensive and still available, at least to build up a few clones.

A dual footprint pcb, if possible, would be fantastic! Maybe we can ask http://www.sowter.co.uk/ for that custom choke?

@ Doug: thanks for input!!

 

[Jean Clochet]

The unit I sold many years ago ended up recently in the USA (after going through several owners I think) and I believe one of the guys who wrote "Recording The Beatles" book has it now.  It had lost its power supply (it was one that I'd built myself, not an original EMI) and had also been messed with a bit along the way so a friend I know was building a new power supply for it and getting it going again - the guy is the one who drew out the schematic I posted here.

Anyway,  I'd mentioned in an email then that I'd always thought it was a nice amp and wouldn't have minded having one around again.  He said that he thought it would be fairly easy to build one and that the anode choke wasn't too critical as long as the inductance was large enough and the capacitance not too great.  He mentioned one by Lundahl as being suitable but I'm sure there are also others too that would work well.

Cutting this story shorter, somewhere on another computer, I have a PCB layout that I'd begun 3 or 4 years ago.  I never finished it as other comittments got in the way but I'm happy to help if any's needed.


Cheers. 

 

[bernbrue]

Cutting this story shorter, somewhere on another computer, I have a PCB layout that I'd begun 3 or 4 years ago.  I never finished it as other comittments got in the way but I'm happy to help if any's needed.
Cheers.

Jean, that would be great. I searched around for the EF37a tube and they are still available. I haven´t found a suitable choke though. It would be nice to have wider range of gain control.
regards
Bernd

 

[Jean Clochet]

I searched around for the EF37a tube and they are still available. I haven´t found a suitable choke though. It would be nice to have wider range of gain control.
regards
Bernd

Hi Bernd,
I just checked my old emails and the Lundahl choke that was mentioned to me is the LL1668 (PDF data attached).  Standing current through the RS61 output valve is quite a bit less than the 25mA that is given on the Lundahl data as resulting in 100H so we can expect that inductance would be higher than 100H.  DCR looks like it is low and capacitance is stated as also being kept low via the winding tecnique used.
I don't know what these cost though?  Maybe they're expensive! 

Regarding gain:  I've never really analysed the RS61 circuit before but, looking at it now, it seems that there is some high frequency boosting with the 330pF cap against the 27K anode load R of the 1st stage and that this is then compensated for to some extent by the relatively low cut off frequency time constant (10.6 KHz) of the 150pF cap and the internal feedback loop's 100K resistor.
Therefore, it might be that the amp as a whole will still remain stable if the main feedback loop gain resistor (820K) is adjusted for lower gains since the internal loop cap is the one determining the dominant pole.  With some rough thumb maths, I'd say it'd at least be OK going down by half voltage or 6dB.  IOW, with a gain resistor of about half the value of 820K, gain will be 6dB lower.  Might be able to go down another 6dB too without worrying about stability but it'd have to be tried empirically.  Might need another compensation cap?

I do know that gain/level was originally changed on these via external pad networks though rather than adjusting loop gains but that never stopped DIYers before! 

EF37a:  Yep, I see them on Fleabay quite often and NOS Mullard or STC don't seem to cost a fortune.  I'm not sure I personally like the EF86 as much as older octal pentodes but they may be close enough in specs to use instead?  Haven't looked though.  The EF40 isn't a bad pentode either but sockets are harder to find.  I think the very first V72 amps used EF40's but not exactly sure of it?

Cheers Bernd

 

[Jean Clochet]

Another thought regarding gain control:
Unless the 1st EF37a is leaky and bad, there isn't any DC on the 820K resistor and the 820K trim pot so, you could just put a log taper panel mount pot in there instead of the 820K gain trim pot (maybe something like a value of 2M ohm) along with a gain limiting resistor (start with 200K and go down if you can) instead of the 820K resistor and you'll have a greater range of adjustment.

 

[dustbro]

I just checked my old emails and the Lundahl choke that was mentioned to me is the LL1668 (PDF data attached).  Standing current through the RS61 output valve is quite a bit less than the 25mA that is given on the Lundahl data as resulting in 100H so we can expect that inductance would be higher than 100H. 

I'm going to get a quote from Sowter and Cinemag to see what they can come up with.
Anyone want to recommend some transformers for In and Out? PCB mount would be nice.
Should have a good idea of costs pretty soon.

 

[dustbro]

looks like the Choke to use is the Sowter 8982 Plate Choke 100 Henry 50 mA
cost is £117.44

 

[Jean Clochet]

looks like the Choke to use is the Sowter 8982 Plate Choke 100 Henry 50 mA
cost is £117.44

That sucker's expensive!  In dollars that's about $185.00
There's a price in $'s for the Lundahl LL1668 from K & K audio and it's $110.00 or, in Sterling, £69.00
It's in a Mu-metal sheild can but even that's still pricey in my opinion for what's basically a few laminations of metal with a bit of wire wrapped around 

Just curious though, what made you say that the Sowter's the one to use? 

 

[bernbrue]

Maybe a noob question, but is there an alternative not using a choke at all? These prices are exorbitant!
regards
Bernd

 

[Jean Clochet]

Maybe a noob question, but is there an alternative not using a choke at all? These prices are exorbitant!
regards
Bernd

I do think we could find an anode choke for less money - For instance, there are a few people over in Hong Kong that I've seen on Ebay selling anode and grid chokes for decent money.  I bet there's also a company such as Edcor that could supply something. 

But, yes there would be an alternative.  You could use a resistor as the load but you'd need to raise the H.T. voltage for that stage to almost double in order to get the same voltage swing.  There would be differences too with regard to the load line then not being an almost horizontal line which might give a different sound?  A choke also isolates the H.T. supply from the signal voltage. 

A single-ended output transformer that is designed for a few mA of standing current could also be used without the choke and the output blocking cap.

Edit:  If they don't have anode chokes, maybe there's an inexpensive output transformer from Edcor that will accept DC current through the primary.  The cheap transformer could be used just for its primary winding inductance with the secondary winding not connected as the signal output.  Might need some kind of dummy termination on the secondary , not sure?  You would then cap couple the signal from the anode to your "better" quality output transformer

 

[Jean Clochet]

I've never tried these but the auction below has a pair of chokes for £85 (£42.50 ea.) including shipping to Europe from Taiwan which is getting better in price:

http://www.ebay.co.uk/itm/200H-anode-plate-choke-6sn7-6j5-5842-5687-12au7-/260985496015?pt=Vintage_Electronics_R2&hash=item3cc3f2a1cf

I'll keep looking.


Edit:  Mullard EF37a data sheet attached.  Also looks like a 6J7 is very similar in characteristics and could probably be used. 

 

[lassoharp]

we could find an anode choke for less money

But of the same quality is the question.  And if less, how much less.  I imagine the bass response will be the first to suffer as the price drops.  Maybe we need CJ in on this thread for some DIY advice on winding our own plate chokes. 


The real compromise for a clone IMO is lack of the original I/O transformers.  Brian Sowter would probably gladly rev engineer one.  You'd probably have to feed someone knock out drops to get a donor though.  ;D

an inexpensive output transformer from Edcor that will accept DC current through the primary

Unfortunately Edcor has not ventured into that territory yet.  I've posted a formal request a couple of years ago on their forum but have heard of no updates.

 

[bernbrue]

Hi,
I found a very interesting 3stage preamp (The Williamson Preamp) with EF37a tubes. Can we learn/adapt anything from this preamp for our RS61? Otherwise nice article to read.
regards
Bernd

 

[Jean Clochet]

But of the same quality is the question.  And if less, how much less.  I imagine the bass response will be the first to suffer as the price drops.  Maybe we need CJ in on this thread for some DIY advice on winding our own plate chokes. 

Yes, using something of dubious quality would be wrong, I agree.
The bass response is determined by core saturation and the inductance value and impedance in relation the the impedance of the valve so,  if 72H is listed on the schematic, 100H and up would cover it.  Even bigger is OK too in my opinion.
I've no idea what the DCR of the original is but it probably wasn't massive given the inductance value - a V72 choke is over 10K DCR for instance but it's also several hundred henries L.  I see there's also a 5K resistor in series with the choke which would add to the DCR for the voltage dropped to the anode.
Capacitance should really be low for good high end response so that's probably where a cheap unit would fall down.

From the Sowter site, the one suggested above is designed for 100mA DC current which is a LOT  ;D .  I would think the output valve in the RS61 is operated at 7 or 8mA given the 10:1 output transformer ratio.

I do know that Cinemag make an anode choke and 10:1 output transformer for a commercial product but I've no idea on price, the availability or not to us mere mortals or what the model numbers are. 

The real compromise for a clone IMO is lack of the original I/O transformers.  Brian Sowter would probably gladly rev engineer one.  You'd probably have to feed someone knock out drops to get a donor though.  ;D

I have some internal pictures of the RS61 I owned (I think I might have posted one or two here but I'll look around) but I have no idea of the transformer winding data other than the EMI part numbers.  And I only know of 3 original amp units - the two that the guy is trying to sell on the Bay for $60K and the one that I sold and is now in the US. 
We would probably need knock out drops intended for elephants to get hold of one of those    Or the same amount of money it would cost to buy an elephant so we can get that pair from Ebay.


Too bad Edcor don't do chokes.

At present, I'd go for Lundahl or give those on Ebay a try but someone should contact Brian Sowter or Cinemag to see about something that's about 100H, good for 10mA or a bit more and has low capacitance.

 

[Jean Clochet]

Can we learn/adapt anything from this preamp for our RS61? Otherwise nice article to read.
regards
Bernd

Hi Bernd,
I only just saw your post.
Thanks for posting the PDF, as you say, it's a nice article.  I think the Williamson is probably a fine pre to modify for mic use just on its own.  Just hang some transformers on it.
I think if people want to build something close to an RS61, then it's probably better to stick with the circuit as is although I don't know what others might want?
I think I might have scared Dustbro away?  Didn't mean to if I did. 

As for me, if I were going to build a pentode mic amp without sticking to an original schematic like the RS61 or V76 for instance, I'd probably use different pentodes anyway -  most likely D3a's in triode and pentode mode.  I'd start from scratch in other words. 

Anyway, thanks again for the article.

 

[bernbrue]

Hi Jean,
don´t get me wrong, it was not my intention to completely change the circuit. I only thought we could find an alternative to the quite expensive choke. Let´s stick to the original circuit and try to find affordable parts. Would it be possible to start with your almost finished pcb layout? I´ve got some Telefunken 1:5 and 10:1 transformers that might work with this circuit. Tubes shouldn´t be too difficult to find. I would be interested to built a mono channel. Yeah, dustbro, your ideas are very much appreciated.
regards
Bernd

 

[dustbro]

I think I might have scared Dustbro away?  Didn't mean to if I did. 

Not at all!! My "day job" is getting in the way of my gear building at the moment.

Do you have a schematic for the PSU you used in your unit? If not, I'll do something similar to what NYD uses on the MILA.

Trying to find a company that has a power transformer close to what we need that also has an additional winding for +48V. Any leads out there?

 

[Jean Clochet]

Not at all!! My "day job" is getting in the way of my gear building at the moment.

That's good.  Glad your day job is busy and glad too you're still with us 

Do you have a schematic for the PSU you used in your unit? If not, I'll do something similar to what NYD uses on the MILA.

I'm sure I do have a schematic somewhere in a pile.  I could probably come up with a better supply today though than I did 25 years ago.
I'll think about it for a while and post something here for folks to look at and decide.

Trying to find a company that has a power transformer close to what we need that also has an additional winding for +48V. Any leads out there?

Yes I think I do.  I'll need to email someone to get a part number etc.  The company who makes it is the the U.S. so it might be off the radar for European builders.  Then again, the transformer was really cheap when bought in quantities (25 X) and it does have dual primary windings for 120V and 240V AC so it might still be worth it for us over here.
I'll find out the details for you guys over the pond anyway.

Bernd,
no problem at all with using my PCB layout but I'll need to look at it and finish it off.  I don't mind doing one though for the benefit of anyone who wants to go that route.

Cheers fellas.

 

[buildafriend]

wow if the schemo early in the thread is functional then this looks really simple..

 

[bernbrue]

Yes I think I do.  I'll need to email someone to get a part number etc.  The company who makes it is the the U.S. so it might be off the radar for European builders.  Then again, the transformer was really cheap when bought in quantities (25 X) and it does have dual primary windings for 120V and 240V AC so it might still be worth it or us over here.
I'll find out the details for you guys over the pond anyway.

Power transformer wouldn´t be a problem for me, I only need the exact specs. I once did a groupbuy for the PM670 power transformer and honestly I wouldn´t do that again. Too much trouble with shipping overseas, lost parcels, packaging etc. Anyway, this is going to be an exciting project. Let´s keep it rolling.
regards
Bernd